The present invention relates generally to the detection of cortical activity. More particularly, the present invention relates to optical imaging of cortical activity in, for example, a human brain to map areas of vital brain function and to distinguish normal and abnormal areas of the cortex during human neurosurgery.
Neurosurgery focuses primarily upon the complete removal of abnormal brain tissue. Ideally, the neurosurgeon would be able to localize areas of the cortex committed to specific neural functions (i.e., speech cortex, primary motor and sensory areas and so forth) as well as the boundaries of pathological or dysfunctional tissue.
Presently, electroencephalogram (EEG) techniques are used prior to and during surgery (often in conjunction with electrocorticography) to examine the human cortex for the purpose of identifying areas of brain function. Such techniques provide a direct measure of brain electrical activity, in contrast to positron emission topography (PET) scans which look at blood flow and metabolic changes in tissue and computerized axial tomography (CAT) scans which look at tissue density differences, and which are used only in preoperative evaluation of a patient.
EEG techniques typically involve placing an array of electrodes (e.g., 16 silver ball electrodes) upon a surface of the cortex. Although EEG techniques have achieved widespread use in the medical field, such techniques do not provide a map of cortical function in response to external stimuli. A modification of EEG techniques, cortical evoked potentials, can provide some functional mapping. However, this technique fails to provide high spatial resolution.
The most commonly employed method of intraoperative localization of cortical function involves direct electrical stimulation of the cortical surface. Direct cortical stimulation is used to evoke either observed motor responses from specific body parts, or in the awake patient, to generate perceived sensations or to interrupt motor speech output. However, the common direct application of a stimulating current to the cortex runs the risk of triggering seizure activity in the operating room. Because the surgeon has no way of knowing the actual extent of cortex stimulation during direct application of a stimulating current, the extent of the brain tissue associated with a given function is also uncertain.
One limit of EEG techniques is that the size of and the distance between electrodes used to form an EEG array are relatively large with respect to the size of brain cells which compose brain tissue. The electrodes associated with a given cortical area therefore often encompass brain cells associated with more than one functional activity. Consequently, the cortical area of the brain which is producing electrical energy detected by a given electrode of the EEG array cannot be identified with specificity. A greater surface area of a cortex being examined is therefore associated with the control of a stimulated portion of a patient's body.
Such inaccuracies can have a dramatic impact when, for example, EEG techniques are used during brain surgery to treat neurological defects such as intractable epilepsy (i.e., epilepsy which can not be treated with medication). Using EEG techniques to identify a portion of the cortex responsible for the epileptic seizure can, and often does, result in a greater amount of cortical tissue removal than would be necessary to treat the epilepsy.
Despite recognition of the aforementioned inaccuracies, the over-inclusiveness of EEG techniques in identifying neurologically dysfunctional cortical areas has nevertheless been deemed acceptable treatment for disorders such as intractable epilepsy. It would therefore be desirable to provide a tool for neurosurgery which would significantly enhance the resolution of cortical activity mapping.
For a number of years, studies have been performed to identify more precise techniques for mapping cortical activity in mammals. One such study is described in an article by Gary G. Blasdel and Guy Salama entitled "Voltage-sensitive dyes reveal a modular organization in working striate cortex", Nature International Weekly Journal of Science, Vol. 321, No. 6070, Jun. 5, 1986, pp. 579-585. The study described by Blasdel et al. is directed to the use of voltage-sensitive dyes for optically imaging neuronal activity in the striate cortex of a monkey. Generally, the study describes identifying patterns of ocular dominance and orientation selectivity (related to the monkey's vision) by examining large areas of striate cortex known to correspond with the central and parafoveal portion of the monkey's visual field.
While these techniques may provide greater resolution in mapping cortical areas of functional activity, staining the exposed cortex with a voltage sensitive dye represents a relatively high level of intrusion to the cortex. The likelihood that such techniques would receive widespread acceptance for mapping in vivo mammalian brains is therefore rather limited.
Another study which is directed to optical imaging of cortical activity has been described, for example, in an article entitled "Functional architecture of cortex revealed by optical imaging of intrinsic signals,", by Amiram Grinvald, et al, Nature, Vol. 324, Nov. 27, 1986, pp. 361-364. As recognized by Grinvald et al., optical imaging of cortical activity offers advantages over conventional electrophysiological and anatomical techniques. The Grinvald study broadly describes a technique of optically imaging cortical activity using some intrinsic changes in the optical properties of mammalian brain tissue in response to electrical or metabolic activity. As noted in the Grinvald et al. article, intrinsic changes in brain tissue due to electrical or metabolic activity, often referred to as intrinsic signals, can be detected using reflection measurements without the use of the aforedescribed voltage sensitive dyes.
As described in an article "Optical Imaging of Cortical Activity: Real-time imaging using extrinsic dye-signals and high resolution imaging based on slow intrinsic signals", Annu. Rev. Physiol., Lieke, Edmund E., et al., 1989, pp. 51:543.9, intrinsic signals are generally considered to be those changes which occur in the light reflecting properties of brain tissue due to activity in the nerve cells of the brain. There are basically two recognized components of the intrinsic signal. A first component is the change in absorption of light having wavelengths between 500-700 nm due to increased blood flow into an area of intense neuronal activity. The increased blood flow results in increased light absorption by hemoglobin at these wavelengths. A second component involves a change in light absorption at wavelengths in the near infrared region of the spectrum (i.e., above 800 nm) and is also associated with the occurrence of neuronal activity. This second component of the intrinsic signal appears to be due to a combination of movement of ions and water from the extracellular space into neurons, swelling of the cells, shrinkage of the extracellular space and neurotransmitter release.
While the Grinvald et al. article recognizes the advantages of using intrinsic signals to provide a non-intrusive method of imaging the barrel areas of a rat somatosensory cortex, each barrel area responding to stimulation of a single mystacial whisker of the rat, this study does not describe how the imaging technique described could, for example, be applied to real time imaging of a mammalian brain cortex. Because the rise time of a reflected light signal indicative of an intrinsic signal is recognized by Grinvald et al. as being slower than dye related signals, the usefulness of the optical imaging as described therein for real time analysis during surgery to remove a cortical area would appear limited.
In an article entitled "Functional Organization of Primate Visual Cortex Revealed by High Resolution Optical Imaging", Ts'O et al., Science, Vol. 249, pp. 417-420, a charge-coupled device (CCD) camera is disclosed for detecting intrinsic signals of the living monkey striate. However, imaging of a limited striate area in response to intrinsic signals is described as having been achieved via a relatively intrusive technique whereby a stainless steel optical chamber having a glass cover plate and being filled with silicon oil was cemented into a hole of the monkey's skull (see footnote 18, p. 420). Further, adequate cortical images are described as having been obtained by averaging over a 30 minute period to a 45 minute period (see footnote 22 on p. 420).
The foregoing studies relate to functional activities of the cortex and have been useful in furthering knowledge regarding complex function of a mammalian brain. However, these studies do not, for example, specifically address the inaccuracies of current EEG techniques or the development of systems and strategies for assisting in the real time, in vivo imaging of a human cortex to more accurately identify dysfunctional cortical areas during neurosurgery.
Accordingly, there is a need in the prior art for a non-intrusive system and method whereby optical imaging can be used to dynamically map cortical activity and thus precisely distinguish abnormal and normal cortical areas of, for example, the human brain during neurosurgery.